The Basics

There is southwest ground swell in the water with 6 to 8 foot waves, a period of 18 seconds, and 10-wave sets hitting the coast every 20 minutes. It’s sunny and the water is glassy. Frog is standing on the bluff overlooking his favorite local surf spot. He turns and says to his friend, “It should be good in a couple of hours.” What? Why isn’t he dashing out to surf what seems like ideal conditions? Because the tide isn’t right. This spot, like so many of our favorite waves, is very tide sensitive. Too high and the waves don’t break on the outer reef. Too low and the inside reef is exposed and dangerous. A few feet of water makes a big difference.

Now imagine if we added four feet of water on top of the normal tides. Permanently. That’s what future sea level rise predictions suggest could happen.

Coastlines throughout the world are very sensitive to changes in sea level. Sea level changes where waves break, how coasts erode or accrete, the tidal flow in estuaries, and the location and extent of wetlands and coral reefs. And small changes, on the order of inches, can have profound impacts on the coasts we love and work so hard
to protect.

The History of Sea Level Rise

On a geologic time scale, the Earth has seen major changes in sea level throughout history. The oceans rose and fell hundreds of meters, alternately flooding and exposing vast extents of the land. That’s why you can find shells in the rocks in Wyoming. About 20,000 years ago, during the last glacial age sea level was about 400 feet below current levels and it has been rising gradually every since.

Changes in sea level are controlled by the volume of water in the sea (eustatic change) and the relative movement of the land (tectonic change). The volume of ocean water is controlled by thermal expansion and by the amount of water trapped as ice. Thermal expansion occurs when global temperatures increase and the water expands. Even the smallest expansion of the seawater can lead to significant changes when it occurs across entire oceans. In additional to thermal expansion, sea level also rises in response to the melting of mountain glaciers and the world’s massive ice sheets in Greenland and Antarctica. A recent paper published in the Journal of Geophysical Research: Oceans found that sea level rise is about half due to melting ice and half due to ocean warming, including 13% from the deepest oceans.[1]

Tectonic changes are more local in effect. Depending on the geologic setting, in some parts of the world the land is rising and in other parts it is sinking. For example, in many areas on the north latitudes such as Canada, Alaska and Scandinavia, the land is actually rebounding from the weight of ancient glaciers and is rising. If the land rises more quickly than global sea level is rising, then the local effect is actually a relative sea level decline. In other places, like the Gulf coast where the land is sinking from resource extraction, sea level rise is exacerbated.

For these reasons, sea level rise will be different at different locations.

Today’s Sea Level Changes

According to the Intergovernmental Panel on Climate Change (IPCC), the average rise in global sea levels has been 1.7 mm/year over the last 100 years.

At a local scale, sea level changes every hour due to many factors including the tides, winds, surf conditions, water temperature and atmospheric pressure. This “noise” in combination with the fact that annual sea level rise in on the order of millimeters make overall changes in sea level impossible to observe with the naked eye. That said, sea level changes over time can be measured using tide gauges and satellite altimetry. For example, this historic tide chart for Honolulu, HI shows annual and decadal fluctuations but also a general trend of sea level rise over the last 100 years.

This historical information, based on everyday observations, makes it pretty clear that sea level is rising. Predicting future sea level increases or a rate of acceleration of future sea level rise is more challenging.

Future Sea Level Rise

Figure 2: Past sea level and sea-level projections from 1990 to 2100 based on global mean temperature projections of the IPCC

Given the dynamic and complex nature of fluctuations in the earth’s global temperature, it is much more difficult to predict future sea level rise than to simply measure the sea level rise that we have observed in the past. The big question is how will global climate change affect sea level rise in the future. The IPCC, which is a scientific body designed to bring together scientists from around the world and synthesize information, has provided a range of scenarios for future sea level rise based on climate models. As you can see in Figure 2, one commonly-cited prediction of future global sea level rise ranges from approximately 0.5 to 1.4 meters (1.5 to 4.5 feet) above 1990 levels by 2100. The variation is dependent on how much the earth warms. It is notable that under all circumstances sea level rise is anticipated to accelerate. And there's some evidence these predictions may be low.

NOAA’s Digital Coast Sea Level Rise and Coastal Flooding Impacts Viewer provides online access to several scenarios of future high tides, uncertainty maps, and information on marsh migration, social vulnerability, and flood frequency. These visualization tools can be used to improve understanding of potential impacts from sea level rise and assist planning efforts in coastal communities.

You can search or navigate a Surging Seas interactive map tool provided by Climate Central to see maps of areas below different amounts of sea level rise and flooding, down to neighborhood scale, matched with area timelines of risk. The tool also provides statistics of population, homes and land affected by city, county and state, plus links to factsheets, data downloads, action plans, and embeddable widgets.

Sea level has been changing throughout the earth’s history and has been rising on most parts of the U.S. coastline over the last 100 years. With the predicted increased emissions of greenhouse gases and resultant global warming, sea level rise will increase and the only question that remains is how much and how fast it will rise.

In the United States, almost 90% of the coast is already vulnerable to coastal erosion, which is creating challenges to protect coastal development without destroying our beaches and coastal habitats. Accelerating sea level rise will make those challenges even greater.

What is clear is that we can’t afford to wait for things to get worse before we begin long term planning for our beaches and coastlines. So next time you are checking the tide before a surf, walk on the beach or a visit to the tide pools, think about the highest of the high tides you see and what it would be like if the tide was another 4 feet higher. That is part of the challenge we will face over the next 100 years of coastal protection.

Shoreline Impacts of Sea Level Rise

Many of us cheer on those swells generated by big storms, whether North Pacific winters on the West Coast, Hurricanes in the fall, or Nor’easters along the Atlantic seaboard. But of course our fun waves often come with a price, eroding the shorelines and beaches that offer a buffer between the powerful oceans and the land that we live on. When too much erosion happens too quickly our homes, streets and public works can be threatened or undermined if they were constructed too close to the waves. There is a constant struggle in many places to keep the sea at bay, whether building concrete seawalls and other structures, or dredging up sand from the seafloor to dump on beaches.

Increased Shoreline Erosion

Throughout history the oceans have slowly risen, chewing away at the land, but thanks to warming seas due to climate change, that rise in water levels is predicted to continue and accelerate well into the next century. During storms there are already many areas that suffer from flooded streets and threats of erosion. Imagine raising the water level an additional 2-4 feet and that flooding could be drastically exacerbated. When you have massive storm swells riding on top of those higher water levels, damaging waves are more likely to erode the shore more quickly with devastating affects on shoreline developments. In low-lying areas, like the barrier islands that form much of the East and Gulf coasts, the damage can be particularly heavy as natural sand movements and replenishment cannot keep pace with accelerated erosion.

Most developed coasts and beaches have buildings very close to the sea leaving little room for the ever-expanding ocean. Recent research by scientists at the University of Arizona found that rising seas will have devastating affects on major cities throughout the U.S., saying:

"The Gulf and southern Atlantic coasts will be particularly hard hit. Miami, New Orleans, Tampa, Fla., and Virginia Beach, Va. could lose more than 10 percent of their land area by 2100."

They analyzed land elevations to determine the amount of space that lies within the expected rise in seas, creating a high-resolution map that can be used by communities to help their future planning in light of their risks. Take a look at what might happen where you live.

This map shows coastal areas in the Southeast that are threatened by different levels of sea level rise. Go to http://climategem.geo.arizona.edu/slr/us48prvi/index.html to find your own state or town on this interactive map to see how your community might fare with predicted sea level rise.

In the face of eroding beaches, owners of beachfront property will often try to use their political influence to demand that "something be done." The intelligent action would be to move the building away from the ocean. Unfortunately, what has often been done in the past has been to armor the coastline with rocks, concrete and steel. This does not protect or maintain the beach - it only protects the buildings. When protective seawalls are built, the beach in front of them will slowly narrow in the face of sea level rise, eventually leading to a complete loss of that beach.

Loss of Wetlands and Flooding

We saw during Hurricanes Katrina and Sandy the impacts that wetland loss played by not having that natural “sponge” to absorb the higher water levels. As sea levels continue to rise we are likely to lose even more wetlands and marshes making the impacts of storms on the coast even worse. Wetlands provide habitat for many species, play a key role in nutrient uptake, serve as the basis for many communities’ economic livelihoods, provide recreational opportunities, and protect local areas from flooding. They can naturally migrate with changing conditions over long periods of time. As the sea rises, the outer boundary of these wetlands will erode, and new wetlands will form inland as previously dry areas are flooded by the higher water levels. But if things change too quickly these critical habitats will get squeezed out of the picture. This can lead to loss of not just natural protection from storms, but also important nursery areas for many types of fish and seafood.

Many barrier islands are backed by large lagoons and sounds. The water levels here will also rise, threatening to flood the islands not just from the ocean side, but from behind. This “inundation” can swamp homes, streets and sewer infrastructure causing permanent land loss and incredible pollution problems. We saw many examples of this because of Hurricane Sandy. There is also the risk of saltwater tainting our water supplies by intruding into the groundwater.

Conclusion

It is critical that community planners make informed decisions when deciding how to react to rising sea levels. The wrong choices could lead down a path where beaches disappear, coastal tourism and fisheries suffer or where billions of dollars are lost to storm-damaged and flooded properties. Fortunately, several states have adopted policies to ensure that beaches, dunes, or wetlands are able to migrate inland as sea level rises. Much more needs to be done, though, and it is upon us to push our decision-makers to plan wisely and make the right decisions for the future of our coast.

Sea Level Rise Adaptation Resources

NOAA’s Digital Coast Sea Level Rise and Coastal Flooding Impacts Viewer provides online access to several scenarios of future high tides, uncertainty maps, and information on marsh migration, social vulnerability, and flood frequency. These visualization tools can be used to improve understanding of potential impacts from sea level rise and assist planning efforts in coastal communities.

NOAA Coastal Services Center Hazards and Climate Adaptation – “helps communities take the steps needed to reduce harm to their communities and the planet. Commonly known as adaptation planning, a wide array of tools and services are available to help communities through this process”

Georgetown Climate Center - Resources for Adaptation Action: materials developed in conjunction with the Georgetown Climate Center to raise awareness and help communities with their planning and educational outreach.

Pacific Institute Study on The Impacts of Sea-Level Rise on the California Coast – “concludes that sea-level rise will inevitably change the character of the California coast, and that adaptation strategies must be evaluated, tested, and implemented if the risks identified in the report are to be reduced or avoided. Populations and critical infrastructure at risk are shown in detailed maps prepared by the Pacific Institute available online here.”

Key Terms

Conferences

Understanding Sea Level Rise in the Face of Extreme Events and an Uncertain Economy, June 18-20, 2013, Fort Lauderdale, FL.

The inaugural Rising Seas Summit brought professionals from national and local government, industry, academic institutions and environmental NGOs together to highlight the interrelationships between sea level rise, climate change and extreme events.